Fix root path lookup in validate-all

[brikis98]

Description

Fixes #1374.

I've updated the runValidateOnAllTerraformModules function, which is used under the hood by ValidateAllTerraformModules and OPAEvalAllTerraformModules, to:

1. Find the Git root using git.GetRepoRootForDirE, instead of a hard-coded ../../ file path.
2. Copy the Git root to a temp folder once, rather than once per sub-test.

Note that I left the code in the test_structure package to keep things backward compatible. We can do that fix some other time.

TODOs

Read the Gruntwork contribution guidelines.

• Update the docs.
• Run the relevant tests successfully, including pre-commit checks.
• Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• Include release notes. If this PR is backward incompatible, include a migration guide.

Release Notes (draft)

Added / Removed / Updated [X].

Migration Guide

[denis256]

Failed tests seems unrelated

[brikis98]

Thanks for the review! Merging.